There is a method of ultrasonic inspection that inspects an internal condition of an inspection target by generating ultrasonic vibration in the inspection target by irradiating laser beam on the inspection target, and analyzing the ultrasonic vibration that propagates in the inspection target.
Further, a technique is being proposed that predeterminedly calculates a frequency by which a base metal resonates, scans a position where ultrasonic vibration is to be generated and a position where the ultrasonic vibration is to be detected, calculates a dimension of a region where the base metal resonates at a lower frequency than its resonating frequency from the scanned positions, and sets the calculated dimension of the region as a dimension of a welded portion.
However, there are risks by which the method of ultrasonic inspection becomes burdensome, and an ultrasonic inspection device becomes complicated.
Due to this, a development of a technique that can easily detect the dimension of the welded portion has long been desired.